Currently within the optical security industry there exists the dual pressures of creating optically variable designs and effects, which have a level of visual simplicity and uniqueness commensurate with the need for clear and unambiguous public recognition and verification, whilst being immune from simulation by the techniques and technologies accessible to organised crime. The particular focus of this invention is the class of optically variable devices wherein the optical effects are generated by the fundamental mechanism of diffraction (first or zero order) that occurs at an interface or surface on the device that is comprised of embossed surface relief. Devices operating through the first order of diffraction are known within the industry as DOVID's (Diffractive Optical Variable Image Device). The most sophisticated threat to the integrity of a high security DOVID is re-origination or reproduction by the uncontrolled origination technologies used to generate iridescent effects and optical imagery within the decorative foiling industry (e.g. low spec dot-matrix and interferential masking techniques).
As a result origination providers have made efforts to make more complex devices. Examples of this approach are described in WO99/59036 and WO2006061586 in which two holographic generating structures are provided, with regions of one structure being interleaved with regions of the other on a scale which is non-visible to the naked eye such that the two structures appear fully integrated. This produces a secure device with an enhanced and visually distinctive optically variable image switching effect which can be easily verified but which is very difficult to re-originate and counterfeit. However in these cases the two holographic generating structures are originated using the same techniques and both generate diffuse diffraction and therefore the optical effects of the two structures are similar. A limitation of such a device is that the visual contrast between the two optically variable image elements provided by respective holographic image generating structures, is limited solely to the difference in their graphical or pictorial form. If we consider a situation wherein the diffractive replay from each generating structure is subject to an additional common diffusing effect, then there is the possibility that this diffusion may be large enough to cause the two image components to overlap within the angular viewing zone i.e. the two images will be simultaneously visible. The lack of differentiation in optical terms between the two image components (they are both diffuse diffractive) is apt to cause confusion to the untrained authenticator to the extent that the two holographic generating structures may be effectively substituted or simulated by one image generating structure, thus compromising the security value of the device. The most typical source of additional diffusion will be a diffuse illuminating light source or multiple light sources—the worst scenario being that when the device is viewed externally in an overcast day when the illuminating sunlight light has been very diffusely scattered by clouds. Another source of diffusion occurs when the device has been applied by the process of hot stamping to heavily textured substrates such as banknote paper.